1. Field of the Invention
The present invention relates to a broadband light source suitable for a pump module for Raman amplification and a Raman amplifier using such light source.
2. Description of the Background Art
The Raman amplifier Raman-amplifies signal light propagating through an optical waveguide path by supplying pump light to the optical waveguide path. In Raman amplification, it is preferable that the gain be uniform at the signal wavelength band. A technique for expanding the bandwidth of Raman gain by expanding the spectral width of pump light, using nonlinear effect in a pre-fiber is disclosed in T. J. Ellingham et al, “ENHANCED RAMAN AMPLIFIER PERFORMANCE USING NON-LINEAR PUMP BROADENING”, 28th European Conference on Optical Communication (ECOC 2002), 8-12 Sep. 2002, Tech. Dig., 4.1.3 (herein after referred to as “Literature 1”).
In the technique of Literature 1, it was necessary to input the pump light having a high launch power of 800 mW into a pre-fiber in order to expand the spectrum of the pump light. The highest output of a semiconductor laser for pump currently available on the optical communication market is 300-350 mW per chip. Even if polarization multiplexing is done, therefore, launch power of 450-530 mW at most can be incident on the pre-fiber when the deterioration with the passage of time from the beginning of life to the end of life and the loss of the polarization multiplexer, which is generally equal to or more than 0.5 dB, are taken into consideration. Although fiber lasers capable of several W output are on the market, but their reliability is questionable.